Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEMS AND PROCESSES FOR PROirIDING
HYDROGEN TO FUEL CELLS
CROSS REFERENCE TO RELATED APPLICATIONS .
This application claims the benefit of Canadian Patent Application No.
2,324,699, filed
October 27, 2000, and Canadian Patent Application No. 2,324,702, flied October
27, 2000, the
disclosures of which are incorporated herein by reference.
FIELD OF THE DISCLOSURE
The present disclosure relates to a fuel cell-based electrical generation
system that enhances
the efficiency and durability of the fuel cell.
BACKGROUND
Fuel cells provide an environmentally friendly source of electrical current.
One form of fuel
cell used for generating electrical power, particularly for vehicle propulsion
and for smaller scale
stationary power generation, includes an anode channel for receiving a flow of
hydrogen gas, a
cathode channel for receiving a flow of oxygen gas, and a polymer electrolyte
membrane (PEM)
which separates the anode channel from the cathode channel. Oxygen gas which
enters the cathode,
reacts with hydrogen ions, which cross the electrolyte to generate a flow of
electrons.
Environmentally safe water vapor is produced as a byproduct.
External production, purification, dispensing and storage of hydrogen (either
as compressed
gas or cryogenic liquid) requires costly infrastructure, while storing of
hydrogen fuel on vehicles
presents considerable technical and economic barriers. Accordingly, for
stationary power generation,
it is preferred to generate hydrogen from natural gas by steam reforming or
partial oxidation followed
by water gas shift reaction. For fuel cell vehicles using a liquid fuel, it is
preferred to generate
hydrogen from methanol by steam reforming or from gasoline by partial
oxidation or autothennal
reforming, again followed by water gas shift reaction. However, the resulting
hydrogen contains
contaminants, such as carbon monoxide and carbon dioxide impurities, that
cannot be tolerated
respectively by the PEM fuel cell catalytic electrodes in more than trace
levels.
The conventional method of removing residual carbon monoxide from the hydrogen
feed to
PEM fuel cells has been catalytic selective oxidation, which compromises
efficiency as both the
carbon monoxide and a fraction of the hydrogen are consumed by low temperature
oxidation, without
any recovery of the heat of combustion. Palladium diffusion membranes can be
used for hydrogen
purification, but have the disadvantages of delivering purified hydrogen at
low pressure, and also the
use ofrare and costly materials.
Pressure swing adsorption systems (PSA) have the attractive features of being
able to
provide continuous sources of oxygen and hydrogen gas, without significant
contaminant levels. PSA
systems and vacuum pressure swing adsorption systems (VPSA) separate gas
fractions from a gas
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2
mixture by coordinating pressure cycling and flow reversals over an adsorber
or adsorbent bed, which
preferentially adsorbs a more readily adsorbed gas component relative to a
less readily adsorbed gas
component of the mixture. The total pressure of the gas mixture in the
adsorber is elevated while the
gas mixture is flowing through the adsorber from a first end to a second end
thereof, and is reduced
while the gas mixture is flowing through the adsorbent from the second end
back to the first end. As
the PSA cycle is repeated, the less readily adsorbed component is concentrated
adjacent the second
end of the adsorber, while the more readily adsorbed component is concentrated
adjacent the first end
of the adsorber. As a result, a "light" product (a gas fraction depleted in
the more readily adsorbed
component and enriched in the less readily adsorbed component) is delivered
from the second end of
the adsorber, and a "heavy" product (a gas fraction enriched in the more
strongly adsorbed
component) is exhausted from the first end of the adsorber.
Numerous copper-based, CO-selective adsorbents have been disclosed by Rabo et
al (U.S.
Patent No. 4,019,879), Hirai (U.S. Patent No. 4,587,114), Nishida et al. (U.S.
Patent No. 4,743,276),
Tajima et al. (IJ.S. Patent No. 4,783,433), Tsuji et al. (U.S. Patent No.
4,914,076), Xie et al. (U.S.
Patent No. 4,917,711), Golden et al. (U.S. Patent Nos. 5,126,310; 5,258,571;
and 5,531,809), and
Hable et al. (U.S. Patent No. 6,060,032). Use of some such CO-selective
adsorbents in pressure
swing adsorption processes for removal or concentration of CO has been
commercially established at
industrial scale.
Using certain adsorbents for removing CO from reformate for PEM fuel cells has
been
investigated by researchers at the Argonne National Laboratory, as reported in
the 1998 annual report
of the Fuel Cells for Transportation Program of the U. S. Department of
Energy, Office of Advanced
Transportation Technologies. Bellows (U.S. Patent No. 5,604,047) discloses
using selected noble
metals, and the carbides and nitrides of certain metals, as carbon monoxide
adsorbents in a steam
displacement purge cycle for removing CO from reformate feed to fuel cells.
However, the conventional system for implementing pressure swing adsorption or
vacuum
pressure swing adsorption uses two or more stationary adsorbers in parallel,
with directional valuing
at each end of each adsorber to connect the adsorbers in alternating sequence
to pressure sources and
sinks. This system is cumbersome and expensive to implement due to the large
size of the adsorbers
and the complexity of the valuing required. Further, the conventional PSA
system use of applied
energy inefficiently because of irreversible gas expansion steps as adsorbers
are cyclically pressurized
and depressurized within the PSA process. Conventional PSA systems could not
be applied to fuel
cell power plants for vehicles, as such PSA systems are far too bulky and
heavy because of their low
cycle frequency and consequently large adsorbent inventory.
Another problem is the need for air compression with a substantial mechanical
parasitic load
to achieve high power density and high voltage efficiency with PEM fuel cells,
either in the absence
of PSA in prior art fuel cell systems, or to a lesser extent with the use of
PSA to increase oxygen
concentration. If, as usual by the case; mechanical power is provided by an
electric motor powered by
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the fuel cell, significant efficiency losses occur in electrical power
conversion and conditioning for
variable speed compressor drive, and the fuel cell stack must be substantially
larger to support this
parasitic load as well as the application load to which useful power is
delivered. In prior art PEM fuel
cell power plants for automotive and other transportation applications,
approximately 20% of the
gross power output of the fuel cell is diverted to the parasitic load of air
compression:
Yet another problem arises in the need to provide heat for endothermic fuel
processing
reactions to generate low purity reformate hydrogen from hydrocarbon fuels
(e.g. natural gas, gasoline
or diesel fuel) or oxygenate fuels (e.g. methanol, ethanol or dimethyl ether).
In the prior art, the
necessary heat for steam reforming of natural gas or methanol is provided
least in part by burning
hydrogen provided as anode tail gas from the fuel cell. Especially in the case
of methanol reforming,
which can be performed at relatively low temperature, combustion of valuable
hydrogen to generate
such low grade heat is extremely detrimental to overall energetic efficiency.
Likewise, the necessary heat for processing heavier fuels, such as gasoline,
is achieved by
combusting a portion of the fuel in a partial oxidation or autothermal
reforming process. Again, a
portion of the high-grade fuel is consumed to upgrade the remainder of that
fuel to low purity
hydrogen than can be purified for use in the fuel cell. With a low temperature
fuel cell, thermal
efficiency of prior art fizel processing systems has been extremely low, as
high grade fuel is
consumed. No opportunity has been found for efficient thermal integration
between a high
temperature fuel processor and a low temperature fuel cell in transport
applications.
Combined cycle power plants with a gas turbine cycle integrated with a fuel
cell system have
been disclosed. Fuel cell auxiliary power units have been proposed for
automobiles and passenger
railcars with internal combustion engines as primary power plants. PCT Patent
Application
Publication No. WO 00116425 provides examples of how PSA units may be
integrated with gas
turbine power plants, or with fuel'cell power plants having a gas turbine
auxiliary engine.
as
SUMMARY OF THE DISCLOSURE
The disclosed fuel-cell-based electrical generation systems and processes
address the
deficiencies of the prior art fuel cell electrical generation systems. This is
particularly true for
purification of reformate hydrogen, energy efficient PSA oxygen enrichment,
heat recovery from the fuel
cell stack and/or from combustion of hydrogen PSA tail gas, and thermal
powering of air compression
for the oxygen PSA and of any PSA vacuum pumping so as to minimize the size of
the costly fuel cell
stack while maximizing overall energetic efficiency of energy conversion from
the raw fuel.
In general, the disclosed electrical current generating systems comprise a
fuel cell, an oxygen
gas delivery system, and a hydrogen gas delivery system. The fuel cell can
include an anode channel
having an anode gas inlet for receiving a supply of hydrogen gas, a cathode
channel having a cathode
gas inlet and a cathode gas outlet, and an electrolyte in communication with
the anode and cathode
channel for facilitating ion transport between the anode and cathode channel.
The oxygen gas
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delivery system is coupled to the cathode gas inlet and delivers air or oxygen
(e.g. oxygen enriched
air) to the cathode channel.
The oxygen gas delivery system may simply be an air blower. However, in
certain
embodiments it may incorporate an oxygen pressure swing adsorption system. For
example, a rotary
PSA system can be used comprising a rotary module having a stator and a rotor
rotatable relative to
the stator, for enfiching oxygen gas from air. The rotor includes a number of
flow paths for receiving
adsorbent material therein for preferentially adsorbing a first gas component
in response to increasing
pressure in the flow paths relative to a second gas component. The pressure
swing adsorption system
also may include compression machinery coupled to the rotary module for
facilitating gas flow
through the flow paths for separating the first gas component from the second
gas component.
Described embodiments of the PSA system include a stator having a first stator
valve surface, a
second stator valve surface, and plurality of function compartments opening
into the stator valve
surfaces. The function compartments include a gas feed compartment, a light
reflux exit compartment
and a light reflux return compartment.
In one variation, the compression machinery comprises a compressor for
delivering
pressurized air to the gas feed compartment, and a light reflux expander
positioned between and
fluidly coupled to the light reflux exit compartment and the light reflux
return compartment. A gas
recirculating compressor is coupled to the light reflux expander for supplying
oxygen gas, exhausted
from the cathode gas outlet, under pressure to the cathode gas inlet. As a
result, energy recovered
from the pressure swing adsorption system can be applied to boost the pressure
of oxygen gas
delivered to the cathode gas inlet.
The oxygen gas delivery system is coupled to the cathode gas inlet and
delivers oxygen gas
to the cathode channel. The hydrogen gas delivery system supplies purified
hydrogen gas to the
anode gas inlet, and may recirculate hydrogen gas from the anode gas exit back
to the anode gas inlet
with increased purity so as to avoid accumulation of impurities in the anode
channel.
In one variant ofthe above-described embodiments, the oxygen gas separation
system
comprises an oxygen pressure swing adsorption system, the hydrogen gas
separation system
comprises a reactor for producing a first hydrogen gas feed from hydrocarbon
fuel, and a hydrogen
pressure swing adsorption system is coupled to the reactor for purifying
hydrogen gas received from
the first hydrogen gas feed. Hydrogen gas from the anode exit may be
recirculated to the hydrogen
pressure swing adsorption system as a second hydrogen gas feed. Both pressure
swing adsorption
systems may include a rotary module having a stator and a rotor rotatable
relative to the stator. The
rotor includes a number of flow paths for receiving adsorbent material therein
for preferentially
adsorbing a first gas component in response to increasing pressure in the flow
paths relative to a
second gas component. The function compartments include a gas feed compartment
and a heavy
product compartment.
The feed gas to the hydrogen PSA system is reformate gas or syngas, generated
in alternative
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fuel processing methods lrnown to the art by steam reforming (e.g. of methanol
or natural gas or light
hydrocarbons), or by autothermal reforming or partial oxidation ("POX") (e.g.
of natural gas, gasoline
or diesel fuel). The CO content of methanol reformate (generated by relatively
low temperature steam
reforming of methanol) is typically about 1% or somewhat less. Other fuel
processors (e.g. steam
methane reformers, and POX or autothermal reformers operating on any
feedstock) operate at a much
higher temperature, and preferably include a lower temperature water gas shift
reactor stage to reduce
to CO content to about 1 % or less.
The reformate gas contains hydrogen plus the basic impurity components of CO2,
CO and
water vapor. If generated by air-blown POX or autothermal reforming, the
reformate gas will also
contain a large inert fraction of nitrogen and argon. The fraction of inert
atmospheric gases can be
greatly reduced if an oxygen PSA system is used to supply the POX or
autothermal reformer, either
' directly from the PSA, or as humid and still oxygen enriched air that has
been passed through the fuel
cell cathode channel, which was directly fed oxygen-enriched air from the PSA.
In one variation, the oxygen pressure swing adsorption system includes a
compressor
coupled to the gas feed compartment for delivering pressurized air to the gas
feed compartment, and a
vacuum pump coupled to the compressor for extracting nitrogen product gas from
the heavy product
compartment. The hydrogen reactor comprises a steam reformer, including a
burner, for producing
syngas, and a water gas shift reactor coupled to the steam reformer for
converting some CO to
hydrogen. The hydrogen pressure swing adsorption system includes a vacuum pump
for delivering
fuel gas from the heavy product compartment to the burner. The fuel gas is
burned in the burner, and
the heat generated therefrom is used to supply the endothermic heat of
reaction necessary for the
steam reformer reaction. The resulting reformate gas is delivered to the water
gas shift reactor for
removal of impurities, and then delivered as the impure hydrogen gas feed to
the hydrogen pressure
swing adsorption system.
In another variation, the invention includes a burner for burning fuel. The
reactor comprises
an autothermal reformer for producing syngas, and a water gas shift reactor
coupled to the
autothermal reformer for converting the syngas to the impure hydrogen gas
feed. The compressor of
the oxygen pressure swing adsorption system delivers pressurized air to the
burner, and the heavy
product gas is delivered from the hydrogen pressure swing adsorption system as
tail gas to be burned
in the burner. The compression machine of the oxygen pressure swing adsorption
system also
includes an expander coupled to the compressor for driving the compressor from
hot gas of
combustion emitted from the burner. The feed compressor with the expander may
be on a common
shaft with a motor drive, or may constitute a free rotor similar to an
automotive turbocharger. The
same expander or another expander may be coupled to a vacuum pump to assist
the PSA process.
Again, the vacuum pump with its expander may be provided as a free rotor
similar to an automotive
turbocharger. Heat from the burner may also be applied to preheat air and/or
fuel supplied to the
autothermal reformer.
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Independently of whether PSA is used for oxygen enrichment, the disclosed
processes and
systems provide a hydrogen PSA apparatus for purifying the reformate. The
hydrogen PSA may be
designed to deliver high purity hydrogen, or else may be designed less
stringently to achieve
adequately high removal of noxious components or contaminants (harmful to the
fuel cell) such as
CO, HZS, halogens, methanol, etc. In the latter case, the hydrogen PSA would
in its first pass only
achieve partial removal of less harmful constituents (e.g., N2, Ar and COz).
In that case, anode tail
gas may be recycled to the feed end of the PSA inlet for use in a feed
pressurization step, thus
avoiding any need for mechanical reeompression. Even when high hydrogen purity
is specified for
the PSA, this Feature enables a small bleed from the end of the anode channel
back to the feed
pressurization step of the hydrogen PSA, as would be desirable for avoiding a
strict dead-headed
configuration with the risk of accumulation in the anode channel of any
contaminant slip due to
equipment imperfections or operational transient upsets.
Accordingly, a first embodiment of the disclosed processes and systems
contemplates
providing a hydrogen-containing gas stream that includes carbon monoxide,
introducing the
hydrogen-containing gas stream into a pressure swing adsorption module that
includes at least one
carbon monoxide-selective adsorbent to produce a purified hydrogen-containing
gas stream, and
introducing the purified hydrogen-containing gas stream to the fuel cell
anode. A further disclosed
process and system for providing a hydrogen-containing gas stream to a fuel
cell anode involves
introducing a hydrogen-containing feed gas stream into an adsorption module
having at least a first
adsorbent and at least one second material, and optionally plural materials
selected from a second
adsorbent, a steam reforming catalyst, and a water gas shift reaction
catalyst, wherein the first
adsorbent and the second adsorbent are chemically distinct and at least one of
the first adsorbent or
the second adsorbent preferentially adsorbs a contaminant in the hydrogen-
containing feed gas stream
to produce a purified hydrogen-containing gas stream.
Operating temperature of the adsorbers in the hydrogen PSA unit can be
elevated well above
ambient, as the reformate gas is supplied at a temperature after water gas
shift of typically about
200°G, while operating temperatures of PEM fuel cells may extend from
about 80°C to about 100°C.
Alternatively, the adsorbers may be operated at a lower temperature if the
reformate is cooled, thus
providing an opportunity for partial removal of water and any methanol vapor
by condensation before
admission to the hydrogen PSA unit. Advantages of operation at moderately
elevated temperature are
(1) reformate coolers and water condensers upstream of the hydrogen PSA can be
avoided, (2) PSA
removal of water vapor and COz may be more readily achieved at moderately
elevated temperature
compared to ambient temperature, (3) CO can be more selectively adsorbed than
COz over Cu(I)-
loaded adsorbents, particularly at elevated temperature, and (4) kinetics of
CO sorption and
desorption on CO-selective sorbents may be greatly enhanced at higher
temperature. Consequently,
in certain embodiments the operating temperature range for the adsorbers is
from about 80°C to about
200°C, and a more particular operating range is from about 100°C
to about 160°C. As used herein,
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"operating temperature of the adsorbers" denotes the temperature of a gas
flowing through the
adsorbers and/or the temperature of the adsorber beds.
The hydrogen PSA unit may be configured to support a temperature gradient
along the
length of the flow channels, so that the temperature at the first end of the
adsorbers is higher than the
temperature at the second end of the adsorbers.
Especially for low purity hydrogen with anode recycle, the hydrogen PSA may
use CO-
selective adsorbents with CO-complexing ions, such as Cu(I) or Ag (e.g,
Ag(I)), introduced by ion
exchange or impregnation into a suitable adsorbent carrier or support. Prior
art CO-selective
adsorbents have used a wide diversity of zeolites, alumina or activated carbon
adsorbents as carriers
or polymers as supports. With CO-selective adsorbents, enhanced hydrogen
recovery may be
achieved while tolerating some accumulation of non-CO impurities circulated
through the fuel cell
anode loop.
Potential problems with CO-selective adsorbents used to purify hydrogen from
reformate
include (1) compatibility with water vapor that may deactivate the adsorbent
or cause leaching of
impregnated constituents, (2) over-reduction by hydrogen, causing the CO-
complexing ion to reduce
to inert metallic form, and (3) relatively slow kinetics of CO-complexing as
compared to physical
adsorption.
The active adsorbent in the disclosed processes and systems (such as a CO-
selective
component) can be supported on thin adsorbent sheets, which are layered and
spaced apart by spacers
to define flow channels, thereby providing a high-surface-area, parallel
passage support with minimal
mass transfer resistance and flow channel pressure drop. With crystalline
adsorbents such as zeolites,
and amorphous adsorbents such as alumina gel or silica gel, the adsorbent
sheet is formed by coating
or in-situ synthesis of the adsorbent on a reinforcement sheet of inert
material, e.g. a wire mesh, a
metal foil, a glass or mineral fiber paper, or a woven or nonwoven fabric.
Active carbon adsorbent
may also be coated onto a reinforcement sheet of inert material, but adsorbent
sheets of active carbon
may also be provided as self supporting carbon fiber paper or cloth. Adsorbers
of the layered
adsorbent sheet material may be formed by stacking flat or curved sheets.
Alternatively, adsorbers
may be a spiral roll, with the flow channels between the sheets extending from
the first end of the
adsorber to the second end thereof. The adsorbers generally fill the volume of
the adsorber housing
of the desired shape. Examples of methods and structures with packed, spirally
wound adsorbents are
disclosed in commonly-owned, co-pending U.S. Provisional Application No.
60/285,527, filed April
20, 2001, and incorporated herein by reference. Typical thickness of the
adsorbent sheet may be in
the range of about 100 to about 200 microns, while flow channel spacing
between the sheets may be
in the range of about 50 to about 200 microns.
According to one variation of the disclosed PSA units, the adsorbent material
contacting the
flow channels between the first and second ends of the adsorbers may in
general be selected to be
different in distinct zones of the flow channels, so that the adsorbers would
have a succession of zones
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(e.g. a first zone, a second zone, a third zone, a perhaps additional zones)
with distinct adsorbents
proceeding along the flow channels from the first end to the second end. As an
alternative to distinct
zones of adsorbents, the different adsorbents may be provided in layers or
mixtures that include
varying gradients of adsorbent concentrations along the gas flow path. The
transition from one
adsorbent to another may also be a blended mixture of the two adsorbents
rather than a distinct
transition. A further option is to provide a mixture of the different
adsorbents that may or may not be
homogeneous and such mixture may be combined with a discrete zone or zones.
In a first variant configured to deliver high purity hydrogen, the adsorbent
in a first zone of
the adsorbers adj acent the first end will be a desiccant to achieve bulk
removal of water vapor in that
first zone, the adsorbent in a second zone in the central portion of the
adsorbers will be selected to
achieve bulk removal of COz and some removal of CO, and the adsorbent in a
third zone of the
adsorbers will be selected to achieve final removal of CO and substantial
removal of any additional
inert components, such as nitrogen and argon. A suitable desiccant, without
limitation, for the first
zone is alumina gel. A suitable adsorbent for the second zone is 13X zeolite,
or SA, or active
charcoal. Suitable adsorbents for the third zone, again without limitation,
may be a strongly carbon
monoxide and nitrogen selective adsorbent selected from the group including
but not limited to Na-
LSX, Ca-LSX, Li-LSX, Li- exchanged chabazite, Ca- exchanged chabazite, Sr-
exchanged chabazite.
The zeolite adsorbents of this group are characterized by strong
hydrophilicity, corresponding to
selectivity for polar molecules. This first variant relying on physical
adsorption will operate most
effectively at relatively lower temperatures, unlikely to exceed much more
than about 100°C although
certain adsorbents such as Ca- or Sr-exchanged chabazite would remain
adequately effective for CO
and Nz removal at temperatures to about 1 SO°C.
In a second similar variant also configured to deliver high purity hydrogen,
the adsorbent in
the second or third zone may be a more strongly carbon monoxide selective
adsorbent such as a
Cu(I)-exchanged zeolite. The zeolite may be, for example, be an X-or a Y-type
zeolite, mordenite, or
chabazite. For stability against over-reduction while contacting nearly pure
hydrogen, the
exchangeable ions of the zeolite may be a mixture of Cu(I) and other ions such
as Na, Li, Ca, Sr,
other transition group metals.or lanthanide group metals. The mixed ions may
also or alternatively
include Ag as a minor component for enhanced CO-selectivity.
In a third variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in a first zone of the adsorbers adjacent
the first end will be a
desiccant to achieve bulk removal of water vapor in that first zone, the
adsorbent in a second zone in
the central portion of the adsorbers will be selected to achieve bulk removal
of COz and some removal
of CO, and the adsorbent in a third zone of the adsorbers will be selected to
achieve final removal of
CO and partial removal of any nitrogen and argon. A suitable desiccant for the
first zone, without
limitation, is alumina gel. A suitable adsorbent for the second zone, again
without limitation, is
alumina gel impregnated with Cu(I), or active carbon impregnated with Cu(I).
Suitable adsorbents for
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the third zone may be similar to those used in the second zone, or may be a CO-
and nitrogen
selective adsorbent as in the first or second variants above.
In a fourth variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in some or all zones of the adsorbers will
be a moderately
hydrophobic adsorbent selected from the' group including, but not limited to,
active carbon and Y-
zeolite, and preferably containing Cu(I) for enhanced CO- selectivity in a
zone adjacent the second
end of the adsorbers.
In a fifth variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in some or all zones of the adsorbers will
be a strongly
hydrophobic adsorbent selected from the group including but not limited to
silicalite and
dealuminified Y-type zeolite. The hydrophobic adsorbent may preferably contain
Cu(I) for enhanced
CO selectivity.
In a sixth variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in the first or second zone of the adsorbers
will include a
component catalytically active at the operating temperature of that zone for
the water gas shift
reaction. The catalytically active component may be any known water gas shift
catalyst, e.g. Cu-Zn0
based catalysts. Preferably, the catalytically active component may be metal
carbonyl complexes of a
transition group metal or a mixture of transition group metals (e.g. Cu, Ag,
Ni, Pd, Pt, Rh, Ru, Fe,
Mo, etc.) inserted into the zeolite cages of, for example, an X or Y-type
zeolite. A portion ofthe
carbon monoxide sorbed onto the catalytically active component may then react
with water vapor by
the water gas shift reaction to generate carbon dioxide and additional
hydrogen. It is known [J.J.
Verdonck, P.A. Jacobs, J.B. Uytterhoeven, "Catalysis by a Ruthenium Complex
Heterogenized in
Faujasite-type Zeolites: the Water Gas-shift Reaction", J.C.S. Chem. Comm.,
pp. 181-182, 1979] that
ruthenium complexes stabilized within X or Y zeolites provide greater water-
gas shift catalytic
activity than conventional copper based catalysts. Other water gas shift
catalysts known in the art
include platinum supported on ceria and transition metal carbides. Iron-chrome
catalysts are used for
industrial water gas shift reactions at higher temperatures.
In a seventh variant configured to deliver at least partially purified
hydrogen with CO nearly
completely removed, the adsorbent in the first zone of the adsorbers is an
adsorbent selective at the
elevated operating temperature of the first zone for carbon dioxide in
preference to water vapor.
Suitable such adsorbents known in the art include alkali-promoted materials.
Illustrative alkali-
promoted materials include those containing cations of alkali metals, such as
Li, Na, I~, Cs, Rb, and/or
alkaline earth metals, such as Ca, St, and Ba. The materials typically may be
provided as the
hydroxide, carbonate, bicarbonate, acetate, phosphate, nitrate or organic acid
salt compound of the
alkali or alkaline earth metals. Such compounds may be deposited on any
suitable substrate such as
alumina. Examples of specific materials include alumina impregnated with
potassium carbonate and
hydrotalcite promoted with potassium carbonate. The adsorbent in the second
zone of the adsorbers
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will include a component catalytically active at the operating temperature of
that zone for the water
gas shift reaction, and optionally also for a steam reforming reaction of e.g.
methanol or methane. As
in the sixth variant above, the catalytically active component in the second
zone may be a known
water gas shift or steam reforming catalyst, or may be a transition group
metal dispersed in zeolite
5 cages and reversibly forming a metal carbonyl complex at the operating
temperature of the second
zone. The second or preferably third zone of the adsorbers contains adsorbent
with some useful
working capacity for carbon monoxide and other impurity components at the
operating temperature of
that zone. Because carbon dioxide is strongly adsorbed in the first zone, the
concentration of carbon
dioxide in the second zone is maintained at a reduced level by the PSA
process, while water vapor
10 concentration remains relatively high in the second zone. Hence, in this
seventh variant the water gas
shift reaction equilibrium (and the steam reforming equilibrium if applicable)
is continually shifted by
the PSA process, which continually removes both hydrogen and carbon dioxide
from the catalytically
active second zone while preventing passage of carbon monoxide into the
hydrogen product passing
the third zone, so that essentially all carbon monoxide is consumed to
generate carbon dioxide and
additional hydrogen. This is an example of a PSA reactor or "sorption enhanced
reactor", driving the
water gas shift reaction substantially to completion while achieving adequate
purification of the
hydrogen.
The reforming and/or water gas shift reaction catalysts) described above may
be included in
any part of the adsorber bed, but typically are included in the section prior
to removal of the water
vapor since water vapor is a reactant for the reforming and water gas shift
reactions.
Industrial Hz PSA is normally conducted at considerably elevated pressures (>
10 bara) to
achieve simultaneous high purity and high recovery (~ 80%-85%). Fuel cell
systems operating with
pressurized methanol reformers or integrated with gas turbine cycles may
operate at relatively high
pressures. However, most PEM fuel cell systems operate at ambient to about 3
bars pressure. As
feed pressure and the overall working pressure ratio of the PSA are reduced,
productivity and
recovery of a simple cycle deteriorate. Under given pressure conditions, use
of CO-selective
adsorbents should significantly improve recovery at specified product CO
concentration, if hydrogen
purity with respect to other impurities such as nitrogen and carbon dioxide
can be relaxed.
At very low feed pressures (e.g. 2-3 tiara), the HZ PSA may need supplemental
compression
to achieve high recovery. Vacuum pumping may be used to widen the working
pressure ratio, or
alternatively "heavy reflex," which is recompression and recycle to the PSA
feed of a fraction of its
exhaust stream at full pressure. Vacuum and heavy reflex options may be
combined in PSA systems
for reformate purification. The heavy reflex option using 13X zeolite
adsorbent, which is not
particularly CO-selective achieved ~ 95% recovery from synthetic methanol
reformate at ~3 tiara feed
pressure and atmospheric exhaust without vacuum pumping.
To get heavy reflex in a very low pressure PSA, the vacuum pump may be
configured so that
part of its flow is reinjected into the PSA feed. Extremely high hydrogen
recovery can then be
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obtained (even at a fairly low overall pressure ratio) by pumping enough heavy
reflux. The vacuum
level can be traded against the mass flow of heavy reflux.
A fuel cell may be a standalone power plant, or else it may be integrated with
some type of
combustion engine. In the case of a standalone fuel cell, all mechanical power
for air handling
compression and any oxygen and/or hydrogen PSA units must be provided as
electrical power by the
appropriately sized fuel cell stack. In this case, tight constraints apply to
the recovery level that must
be achieved by the Hz PSA at specified purity. In the absence of any useful
export use for high grade
heat, an efficient heat balance requires that the heating value of combustible
waste gases (H2, CO and
unreacted fuel) be matched to the heat demand of the fuel processor. For a
fuel cell with steam
reforming (e.g. methanol or natural gas), nominal hydrogen recovery by the H~
PSA has to be about
75% to 80% as the PSA tail gas is burned to heat the reformer; while for a POX
or autothermal
reformer, hydrogen recovery by the PSA needs to be extremely high (at least
90% to 95%) as such
reformers can only use a limited amount of external combustion heat from
burning PSA tail gas or
fuel cell anode tail gas, e.g. for preheating feed oxygen/air and fuel
reactants to the reformer.
In order to achieve high process efficiency and high recovery of the PSA units
along with
high overall efficiency of the fuel cell system, the hydrogen PSA tail gas may
be burned in an
auxiliary combustion engine to drive the air handling system compressor and
any vacuum pumps for
the oxygen and hydrogen PSA units. Thus, according to another presently
disclosed embodiment, a
process and system is described that includes providing at least one first
pressure swing adsorption
module that produces an oxygen-enriched gas stream, the first pressure swing
adsorption module
including at least one device selected from a first compressor or first vacuum
pump; providing at least
one second pressure swing adsorption module that produces a purified hydrogen
gas stream and a
separation exhaust gas stream, the second pressure swing adsorption module
including at least one
device selected from a second compressor or second vacuum pump; introducing
the oxygen-enriched
gas stream and the purified hydrogen gas stream into a fuel cell; and
introducing the separation
exhaust gas stream as a fizel into a combustion engine for driving at least
one auxillary device,
typically selected from the first compressor, first vacuum pump, second
compressor, second vacuum
pump, or an electric generator.
For smaller plants, internal combustion engines may be attractive relative to
gas turbine
configurations. Either way, powering the compressor and vacuum pumps) by
burning tail gas avoids
the cost penalty of a bigger fuel cell stack in order to run compression
machinery as parasitic
electrical loads. The engine exhaust heat and/or cooling jacket heat may be
fizrther recovered to
preheat and vaporize fuel reactants and to provide some or all of the heat of
reforming for a methanol
reformer as described below in more detail.
The engine could be a reciprocator or a rotary engine. It may aspirate the
hydrogen PSA tail
gas directly as fixel, or else be turbocharged to pull greater vacuum from the
PSA exhaust. Modern
Wankel derivative engines have favourable specific displacement and power
density. Thus, an
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auxiliary internal combustion engine could act as its own vacuum pump on tail
gas being inducted
directly as fuel. Some oxygen enriched tail gas from the fuel cell cathode
could be fed as a
supplement to intake air to make up for the heavy COz load. In view of the
hydrogen, water and
carbon dioxide content of the tail gas fueling this engine, conditions are
favourable for extremely low
emissions of NOx and other noxious contaminants. Here, the above strict heat
balance constraints on
necessary hydrogen recovery to be achieved by the PSA may be relaxed in
designing for most
desirable technical, emissions and economic performance of the power plant
because tail gas
combustion can thermally power auxiliary compression loads as well as provide
endothermic heat of
fuel processing. The combustion engine may power all compressors and vacuum
pumps for the Oz
PSA, along with vacuum pump and/or heavy reflux compression for the.Hz PSA.
This auxiliary gas
turbine cycle allows a heavy reflux vacuum pump and compressor to be driven by
the turboexpander
which expands the products of hydrogen PSA tail gas combustion. Thus, one
feature of disclosed
processes and systems is integration of the vacuum pumps) with the combustion
engine powered by
tail gas combustion. Either single or multiple spool gas turbine
configurations may be considered in
connection with the combustion engine. Centrifugal or axial machines may be
used as the
compressors and pumps. Approaches based on integration of gas turbines and
fuel cells are
particularly favourable for larger power levels.
Further disclosed embodiments are directed to improved steam reforming
processes
(particularly methanol reforming) when coupled to a fuel cell. The
conventional approach for
methanol reforming is to increase the pressure of liquid reactants to an
elevated pressure for
vaporization and the vapor phase methanol reforming reaction. This approach
enables the reactor
itself to be compact, and provides driving pressure for hydrogen purification
by PSA or palladium
diffusion membranes.
A novel low pressure process is disclosed herein for steam methanol reforming
that can get
enhanced heat recovery from a low pressure fuel cell. More than 60% of the
endothermic heat of
steam reforming methanol is the heat of vaporization to boil the methanol and
the water inputs. If the
fuel cell is cooled to vaporize feed liquid fuel and water at the fuel cell
stack working temperature, the
system may be more efficient due to heat recovery, which liberates hydrogen to
generate electricity
while absorbing about 25% of the stack cooling load. A water-rich mix of 14%
methanol in water
boils at atmospheric pressure and 85°C to generate a 50/50 vapor mix as
required by stoichiometry, or
at a modestly higher temperature with a larger excess of water in the liquid
phase to obtain a small
excess of steam as actually required to ensure low CO concentration. Therefore
the liquid mixture of
water containing a fraction of methanol may be circulated as fuel cell stack
coolant, and then flash
evaporated to generate a methanol-Hz0 vapor mix to be admitted into the
reforming catalyst chamber
at fuel cell system working pressure. If the fuel cell operates at less than
85°C, flash evaporation
likely would be performed under vacuum or else with a higher concentration of
methanol (as also
desirable for antifreeze characteristics for winter conditions) so that only a
fraction ofthe water
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required for methanol reforming is provided by vaporization using fuel cell
stack waste heat. As
higher PEM fuel cell operating temperatures are considered, this approach
becomes more viable as
permitting either atmospheric or higher pressure for flash evaporation, or
else a larger temperature
differential driving heat exchange in the stack coolant channels. Thus, there
is disclosed herein a
process and system that involves providing a fuel cell defining a coolant
passage and an anode inlet
for receiving a hydrogen-containing gas stream; mixing liquid water and a
hydrocarbon fuel stream
resulting in a coolant mixture; introducing the coolant mixture into the
coolant passage of the fuel
cell; vaporizing the coolant mixture to form a steam/fuel vapor mixture;
subjecting the steamlfuel
vapor mixture to reaction conditions sufficient for generating a hydrogen-
containing gas stream; and
introducing the hydrogen-containing gas stream into the fuel cell anode inlet.
Using stack heat recovery to boil the methanol reforming reactants is more
attractive for a
relatively low pressure fuel cell (e.g., operating at a pressure below about 2
bars absolute), unless the
working temperature were greatly increased. If all the steam feed to the
methanol reformer is
generated by stack heat recovery, some mechanical compression of the reformer
reactant vapor
mixture generally is needed except for a very low pressure PEM fuel cell (e.g.
operating at a pressure
below 1.5 bars absolute). Such a very low pressure fuel cell would be expected
to benefit greatly
from PSA Oz enrichment as enabling high power density at low total pressure.
However, vacuum
pumping would then be required for both the oxygen PSA and a hydrogen PSA
unit, particularly to
obtain high recovery of hydrogen in the hydrogen PSA.
An alternative approach within the invention is to operate the fuel cell at
somewhat higher
pressure (e.g. operating at a pressure of about 2 or 3 bars absolute), with
the stack coolant liquid
mixture of water and methanol containing a higher concentration of methanol,
so that the vapor
mixture thus generated contains all the methanol vapor for the methanol
reformer, plus only a portion
of the steam required for reforming that methanol. Supplementary steam is then
generated by an
alternative heat source, for example exhaust heat or cooling jacket heat from
a combustion engine or
turbine used to drive the feed air compressor and any vacuum pumps required to
operate the PSA
equipment.
In the case of a POX or autothermal gasoline fuel processor, the endothermic
heat for the
reforming reaction is generated by burning a portion of the fuel stream within
the reforming reactor.
Hence, there is at most a very limited opportunity for burning the hydrogen
PSA tail gas usefully to
support the reforming process (e.g. to preheat incoming air and fuel streams),
because ample high
grade heat is generated within POX and autothermal reformers. If there is no
other use for
combustion heat from burning the hydrogen PSA tail gas, the hydrogen PSA
achieves extremely high
hydrogen recovery (in the range of e.g. 90% to 99%) to achieve heat balance
and full utilization of
fuel. In the case of a methanol reforner with stack heat recovery to boil the
reactants as provided
above within the present invention, the hydrogen PSA would have to achieve
very high hydrogen
recovery (~ 90%) in view of the substantial heat recovery from the stack to
reduce the methanol
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reformer heat demand.
An auxiliary combustion engine or turbine therefore is disclosed, cooperating
with the fuel
cell power plant to at least assist the feed air compression and any vacuum
pumping loads. Tail gas
from the hydrogen PSA unit is now usefully consumed as fuel for the auxiliary
combustion engine or
turbine, so that the necessary hydrogen recovery achieved by the PSA unit may
be relaxed to the
range of e.g. 70% to 90% as the heat balance and fuel utilization constraints
are opened. Hence, the
need for heavy refiux compression and vacuum pumping to assist the hydrogen
PSA unit is reduced or
eliminated. Simultaneously, the auxiliary combustion engine or turbine unloads
the PSA compression
and any vacuum pumping load from the fuel cell electrical output, thus
reducing the size and cost of
the fuel cell.
The thermally integrated combination of the auxiliary combustion engine or
turbine with the
fuel processor provides alternative waste heat sources for vaporizing steam
directly at the reforming
pressure, for heating an endothermic reactor, and for recovering exothermic
heat e. g. of water gas
shift. A thermally integrated design can also be configured to minimize
thermal inefficiencies, e.g. of
heat loss by conduction to the environment, simply by placing hot components
of the fuel processor
and the auxiliary heat engine within a common housing, and with components at
similar operating
temperatures in close adjacent proximity.
The foregoing features and advantages will become more apparent from the
following -
detailed description of several embodiments that proceeds with reference to
the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an axial section of a rotary PSA module.
FIGS. 2 through SB show transverse sections of the module of FIG. 1.
FIG. 6 is a simplified schematic of a fuel cell power plant with a steam
reforming fuel
processor, a PSA unit for reformate hydrogen purification by at least removal
of CO, and a VPSA unit
for oxygen enrichment.
FIG. 7 is a simplified schematic of an additional embodiment of a fuel cell
power plant that
includes a modified steam reforming fuel processor.
FIG. 8 is a simplified schematic of another embodiment of a fuel cell power
plant that
includes a vacuum pump.
FIG. 9 is a simplified schematic of a further embodiment of a fuel cell power
plant that
includes an internal combustion engine and a modified fuel cell stack.
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DETAILED DESCRIPTION OF SEVERAL EMBODIMENTS
One embodiment of an oxygen-enrichment rotary PSA module for use with the
described
methods and systems is described below in connection with FIGS. 1-5B, but the
same or similar
5 rotary PSA module configuration could be used for hydrogen enrichment (i.e.,
separation) in the
disclosed electrical current generating systems. As used herein, a "rotary
PSA" includes, but is not
limited to, either a PSA wherein the adsorbent bed rotates relative to a fixed
valve face or stator or a
PSA wherein the valve face or stator rotates relative to a fixed adsorbent
bed.
FIG. 1 shows a rotary PSA module 1, which includes a number "N" of adsorbers 3
in
10 adsorber housing body 4. Each adsorber has a first end 5 and a second end
6, with a flow path
therebetween contacting a nitrogen-selective adsorbent (for oxygen
enrichment). The adsorbers are
arrayed about axis 7 of the adsorber housing body. The housing body 4 is in
relative rotary motion
about axis 7 with first and second functional bodies 8 and 9, being engaged
across a first valve face
10 with the first functional body 8 to which feed gas mixture is supplied and
from which the heavy
15 product is withdrawn, and across a second valve face 11 with the second
functional body 9 from
which the light product is withdrawn.
In embodiments as particularly depicted in FIGS. 1-5, the adsorber housing 4
rotates and
shall henceforth be referred to as the adsorber rotor 4, while the first and
second functional bodies are
stationary and together constitute a stator assembly IZ ofthe module. The
first functional body shall
henceforth be referred to as the first valve stator 8, and the second
functional body shall henceforth be
referred to as the second valve stator 9. In other embodiments, the adsorber
housing may be
stationary, while the first and second function bodies may be the rotors of
rotary distributor valves.
In the embodiment shown in FIGS. 1-5, the flow path through the adsorbers is
parallel to
axis 7, so that the flow direction is axial, while the first and second valve
faces are shown as flat
annular discs normal to axis 7. However, more generally the flow direction in
the adsorbers may be
axial or radial, and the first and second valve faces may be any figure of
revolution centred on axis 7.
The steps of the process and the functional compartments to be defined will be
in the same angular
relationship regardless of a radial or axial flow direction in the adsorbers.
FIGS. 2-5 are cross sections of module 1 in the planes defined by arrows 12'-
13', 14'-15', and
16'-1T. Arrow 20 in each section shows the direction of rotation of the rotor
4. FIG. 2 shows section
12'-13' across FIG. 1, which crosses the adsorber rotor. In this example, "N"
= 72. The adsorbers 3
are mounted between outer wall 21 and inner wall 22 of adsorber wheel 208.
Each adsorber 3
comprises a rectangular flat pack of adsorbent sheets 23, with spacers 24
between the sheets to define
flow channels here in the axial direction. Separators 25 are provided between
the adsorbers to fill
void space and prevent leakage between the adsorbers. In other configurations,
the adsorbent sheets
may be formed in curved packs or spiral rolls.
Satisfactory adsorbent sheets have been made by coating a slurry of zeolite
crystals with
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binder constituents onto the reinforcement material, with successfizl examples
including nonwoven
fibreglass scrims, woven metal fabrics, and expanded aluminium foils. The
adsorbent sheets comprise
a reinforcement material, in preferred embodiments glass fibre, metal foil or
wire mesh, to which the
adsorbent material is attached with a suitable binder. For applications such
as hydrogen purification,
some or all of the adsorbent material may be provided as carbon fibers, in
woven or nonwoven form
to serve as its own reinforcement material. Spacers 24 are provided by
printing or embossing the
adsorbent sheet 23 with a raised pattern, or by placing a fabricated spacer
between adj acent pairs of
adsorbent sheets. Alternative satisfactory spacers 24 have been provided as
woven metal screens,
non-woven fibreglass scrims, and metal foils with etched flow channels in a
photolithographic pattern.
Typical experimental sheet thicknesses have been 150 microns, with spacer
heights in the
range of 100 to 150 microns, and adsorber flow channel length approximately 20
cm. Using X-type
zeolites, excellent performance has been achieved in oxygen separation from
air and hydrogen
purification from reformate at PSA cycle frequencies in the range of 1 at
least to 150 cycles per
minute particularly at least 25 cycles per minute.
As shown in FIG. 1, the adsorbers 3 may comprise a plurality of distinct zones
between the
first end 5 and the second end 6 of the flow channels. FIG. 1 illustrates a
first zone 26 adjacent the
first end 5, a second zone 27 in the middle of the adsorbers, and a third zone
28 adjacent the second
end 6. These zones may be entirely distinct as to the local composition of
adsorbent (including any
catalyst), or else may be blended with a continuous gradient of adsorbent
composition. Fewer or
more zones may be provided as desired. The first zone typically contains an
adsorbent or desiccant
selected for removing very strongly adsorbed components of the feed gas
mixture, such as water or
methanol vapor, and some carbon dioxide. The second zone contains an adsorbent
typically selected
for bulk separation of impurities at relatively high concentration, and the
third zone contains an
adsorbent typically selected for removing impurities at relatively low
concentrations.
In embodiments with multiple zones, the volume of each zone may be preselected
to achieve
a desired result. For example, with a 3-zone embodiment the first zone may be
the first 10% to 20%
of the flow channel length from the first end, the second zone may be the next
roughly 40% to 50% of
the channel length, and the third zone the remainder. In embodiments with only
two adsorber zones,
the first zone may be the first 10% to 30% of the flow channel length from the
first end, and the
second zone the remainder. The zones may be formed by coating the different
adsorbents onto the
adsorbent support sheet material in bands of the same width as the flow
channel length of the
corresponding zone. The adsorbent material composition may change abruptly at
the zone boundary,
or may be blended smoothly across the boundary. Particularly in the first zone
of the adsorber, the
adsorbent must be compatible with significant concentrations of water vapor.
For air separation to produce enriched oxygen, alumina gel may be used in the
first zone to
remove water vapor, while typical adsorbents in the second and third zones are
X, A or chabazite type
zeolites, typically exchanged with lithium, calcium, strontium, magnesium
and/or other cations, and
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with optimized silicon/aluminium ratios as well known in the art. The zeolite
crystals are bound with
silica, clay and other binders, or self bound, within the adsorbent sheet
matrix.
In a first variant configured to deliver high purity hydrogen, the adsorbent
in a first zone of
the adsorbers adjacent the first end will be a desiccant to achieve bulk
removal of water vapor in that
first zone, the adsorbent in a second zone in the central portion of the
adsorbers will be selected to
achieve bulk removal of COz and some removal of CO, and the adsorbent in a
third zone of the
adsorbers will be selected to achieve final removal of CO and substantial
removal of any nitrogen and
argon. A suitable desiccant for the first zone is alumina gel. Illustrative
suitable adsorbents for the
second zone are 13X zeolite, or SA, or active charcoal. Suitable adsorbents
for the third zone may be
a strongly carbon monoxide and nitrogen selective adsorbent selected from the
group including, but
not limited to, Na-LSX, Ca-LSX, Li-LSX, Li- exchanged chabazite, Ca- exchanged
chabazite, Sr-
exchanged chabazite. The zeolite adsorbents of this group are characterized by
strong hydrophilicity,
corresponding to selectivity for polar molecules. This first variant relying
on physical adsorption will
operate most effectively at relatively lower temperatures, unlikely to exceed
much more than about
100°C, although certain adsorbents such as Ca- or Sr-exchanged
chabazite remain adequately
effective for CO and Nz removal at temperatures up to about 150°C.
In a second similar variant also configured to deliver high purity hydrogen,
the adsorbent in
the second or third zone may be a more strongly carbon monoxide selective
adsorbent such as a
Cu(I)-exchanged zeolite. The zeolite may for example be an X or Y-type
zeolite, mordenite, or
chabazite. For stability against over-reduction while contacting nearly pure
hydrogen, the
exchangeable ions of the zeolite may be a mixture of Cu(I) and other ions such
as Na, Li, Ca, Sr,
other transition group metals or lanthanide group metals. The mixed ions may
also or alternatively
include Ag(I) as a minor component for enhanced. CO-selectivity.
In a third variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in a first zone of the adsorbers adjacent
the first end will be a
desiccant to achieve bulk removal of water vapor in that first zone, the
adsorbent in a second zone in
the central portion of the adsorbers will be selected to achieve bulk removal
of COz and some removal
of CO, and the adsorbent in a third zone of the adsorbers will be selected to
achieve final removal of
CO and partial removal of any nitrogen and argon. A suitable desiccant for the
first zone is alumina
gel. A suitable adsorbent for the second zone is alumina gel impregnated with
Cu(I), or active carbon
impregnated with Cu(I). Suitable adsorbents for the third zone may be similar
to those used in the
second zone, or may be a CO- and nitrogen selective adsorbent as in the first
or second variants
above.
In a fourth variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in some or all zones of the adsorbers will
be a moderately
hydrophobic adsorbent selected from the group including but not limited to
active carbon and Y-
zeolite, and preferably containing Cu(I) for enhanced CO- selectivity in a
zone adjacent the second
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end of the adsorbers.
In a fifth variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in some or all zones of the adsorbers will
be a strongly
hydrophobic adsorbent selected from the group including but not limited to
silicalite and
dealuminified Y-zeolite. The hydrophobic adsorbent may preferably contain
Cu(I) for enhanced CO-
selectivity.
In a sixth variant configured to deliver at least partially purified hydrogen
with CO nearly
completely removed, the adsorbent in the first or second zone of the adsorbers
will include a
component catalytically active at the operating temperature of that zone for
the water gas shift
reaction. The catalytically active component may be any water gas shift
catalyst, e.g. Cu-Zn0 based
catalysts. Preferably, the catalytically active component may be metal
carbonyl complexes of a
' transition group metal or a mixture of transition group metals (e.g. Cu, Ag,
Ni, Pd, Pt, Rh, Ru, Fe,
Mo, etc.) inserted into the zeolite cages of e.g. an X or Y zeolite. A portion
of the carbon monoxide
sorbed onto the catalytically active component may then react with water vapor
by the water gas shift
reaction to generate carb on dioxide and additional hydrogen.
In a seventh variant configured to deliver at least partially purified
hydrogen with CO nearly
completely removed, the adsorbent in the first zone of the adsorbers is an
adsorbent selective at the
elevated operating temperature of the first zone for carbon dioxide in
preference to water vapor.
Suitable such adsorbents known in the art include alumina impregnated with
potassium carbonate, and
hydrotalcite promoted with potassium carbonate. The adsorbent in the second
zone of the adsorbers
will include a component catalytically active at the operating temperature of
that zone for the water
gas shift reaction and if desired a steam reforming reaction. As in the sixth
variant above, the
catalytically active component in the second zone may be a known water gas
shift catalyst, or may be
a transition group metal dispersed in zeolite cages and reversibly forming a
metal carbonyl complex at
the operating temperature of the second zone. The second or preferably third
zone of the adsorbers
contains adsorbent with some useful working capacity for carbon monoxide and
other impurity
components at the operating temperature of that zone. The third zone of the
adsorbers preferably
contains an adsorbent with useful working capacity for water vapor at the
operating temperature of
that zone. Because carbon dioxide is strongly adsorbed in the first zone, the
concentration of carbon
dioxide in the second zone is maintained at a reduced level by the PSA
process, while water vapor
concentration remains relatively high in the second zone. Hence, in this
seventh variant the water gas
shift reaction equilibrium is continually shifted by the PSA process which
continually removes both
hydrogen and carbon dioxide from the catalytically active second zone while
preventing passage of
carbon monoxide into the hydrogen product passing the third zone, so that
essentially all carbon
monoxide is consumed to generate carbon dioxide and additional hydrogen. The
water gas shift
reaction is thus driven substantially to completion, while achieving adequate
purification of the
hydrogen.
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The water gas shift reaction is exothermic, and consequently heat should be
removed from
the reactive second zone of the adsorbers in which the catalyst is contained.
As disclosed in
copending patent application PCT publication WO 00/76629, the disclosure of
which is incorporated
herein by reference, the adsorber housing may be configured as a heat
exchanger so that heat may be
transferred through the containment walls of the individual adsorbers. Heat
may also be removed by
allowing the temperature of the second zone to rise above the temperature of
the first ends of the
adsorbers so that heat is removed by the carbon dioxide product of reaction
and by axial conduction
through the preferably metallic support of the adsorbent laminate, and/or by
allowing the temperature
of the second zone to rise above the temperature of the second ends of the
adsorbers so that heat is
removed by the hydrogen product as sensible heat and by axial conduction
through the preferably
metallic support of the adsorbent laminate.
The above described seventh variant may be readily adapted for the important
application of
steam reforming methanol. The adsorbent in the first zone may be promoted
hydrotalcite, which
preferentially adsorbs carbon dioxide. The catalyst in the second zone may be
any catalyst active for
the methanol steam reforming and water gas shift reactions, e.g. Cu-Zn0 or a
noble metal catalyst.
The adsorbent in the third zone is selective for water and methanol vapor.
Consequently, the
concentration of carbon dioxide is depressed, while the concentrations of
steam and methanol vapor
are elevated, over the second zone so as to shift the reaction equilibria for
high conversion of
methanol and removal of carbon monoxide by water gas shift. At a given
temperature, the reaction
rate will be enhanced compared to the same catalyst in a conventional reactor.
The vapor phase steam reforming reaction is endothermic, and consequently heat
must be
provided to the reactive second zone of the adsorbers in which the catalyst is
contained. As disclosed
in our copending patent application PCT publication WO 00/76629, the
disclosure of which is
incorporated herein by reference thereto, the adsorber housing may be
configured as a heat exchanger
so that heat may be transferred through the containment walls of the
individual adsorbers. Heat may
also be supplied by allowing the temperature of the second zone to fall below
the temperature of the
first ends of the adsorbers so that heat is delivered to the second zone as
sensible heat of the reactants
and also by axial conduction through the preferably metallic support of the
adsorbent laminate from
the first end of the adsorbers.
FIG. 3 shows the porting of rotor 4 in the first and second valve faces
respectively in the
planes defined by arrows 14'-15', and 16'-1T. An adsorber port 30 provides
fluid communication
directly from the first or second end of each adsorber to respectively the
first or second valve face.
FIGS. 4A and 4B show the first stator valve face 100 of the first stator 8 in
the first valve
face 10, in the plane defined by arrows 14-15. Fluid connections are shown to
a feed compressor 101
inducting feed gas through inlet filter 102, and to an exhauster 103
delivering second product to a
second product delivery conduit 104. Compressor 101 and exhauster 103 are
shown coupled to a
drive motor 105.
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~ a a ~G~iWe s s oo s
Arrow 20 indicates the direction of rotation by the adsorber rotor. In the
annular valve face
between circumferential seals 106 and 107, the open area of first stator valve
face 100 ported to the
feed and exhaust compartments is indicated by clear angular segments 111-116
corresponding to the
first functional ports communicating directly to functional compartments
identified by the same
5 reference numerals 111-116. The substantially closed area of valve face 100
between functional
compartments is indicated by hatched sectors 118 and 119 which are slippers
with zero clearance, or
preferably a narrow clearance to reduce friction and wear without excessive
leakage. Typical closed
sector 118 provides a transition for an adsorber, between being open to
compartment 114 and open to
compartment 115. Gradual opening is provided by atapering clearance channel
between the slipper
10 and the sealing face, so as to achieve gentle pressure equalization of an
adsorber being opened to a
new compartment. Much wider closed sectors (e.g. 119) are provided to
substantially close flow to or
from one end of the adsorbers when pressurization or blowdown is being
performed from the other
end.
The feed compressor provides feed gas to feed pressurization compartments 111
and 112,
15 and to feed production compartment 113. Compartments 111 and 112 have
successively increasing
working pressures, while compartment 113 is at the higher working pressure of
the PSA cycle.
Compressor 101 may thus be a multistage or split stream compressor system
delivering the
appropriate volume of feed flow to each compartment so as to achieve the
pressurization of adsorbers
through the intermediate pressure levels of compartments 111 and 112, and then
the final
20 pressurization and production through compartment 113. A split stream
compressor system may be
provided in series as a multistage compressor with interstage delivery ports;
or as a plurality of
compressors or compression cylinders in parallel, each delivering feed air to
the working pressure of a
compartment 111 to 113. Alternatively, compressor 101 may deliver all the feed
gas to the higher
pressure, with throttling of some of that gas to and 112 at their respective
intermediate pressures.
Similarly, exhauster 103 exhausts heavy product gas from countercurrent
blowdown
compartments 114 and 1 I5 at the successively decreasing working pressures of
those compartments,
and finally from exhaust compartment 116, which is at the lower pressure of
the cycle. Similarly to
compressor 101, exhauster 103 may be provided as a multistage or split stream
machine, with stages
in series or in parallel to accept each flow at the appropriate intermediate
pressure descending to the
lower pressure.
In the example embodiment of FIG. 4A, the lower pressure is ambient pressure,
so exhaust
compartment 116 communicates directly to heavy product delivery conduit 104.
Exhauster 103 thus
is an expander which provides pressure letdown with energy recovery to assist
motor 105 from the
countercurrrent blowdown compartments 114 and 115. For simplicity, exhauster
103 may be
replaced by throttling orifices as countercurrent blowdown pressure letdown
means from
compartments 114 and 115.
In some embodiments, the lower pressure of the PSA cycle is subatmospheric.
Exhauster
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103 is then provided as a vacuum pump, as shown in FIG. 4B. Again, the vacuum
pump may be
multistage or split stream, with separate stages in series or in parallel, to
accept countercurrent
blowdown streams exiting their compartments at working pressures greater than
the lower pressure
which is the deepest vacuum pressure. In FIG. 4B, the early countercurrent
blowdown stream from
compartment 114 is released at ambient pressure directly to heavy product
delivery conduit 104. If,
for simplicity, a single stage vacuum pump were used, the countercurrent
blowdown stream from
compartment 115 is throttled down to the lower pressure over an orifice to
join the stream from
compartment 116 at the inlet of the vacuum pump.
If the feed gas is provided at an elevated pressure at least equal to the
higher pressure of the
PSA cycle, as may conveniently be the case of a hydrogen PSA operating with
e.g. methanol
reformate feed, compressor 101 would be eliminated. To reduce energy losses
from irreversible
throttling over orifices to supply feed pressurization compartments e.g. 111,
the number of feed
pressurization stages may be reduced, sot that adsorber repressurization is
largely achieved by product
pressurization, by backflll from light reflux steps. Alternatively, compressor
101 may be replaced in
part by an expander which expands feed gas to a feed pressurization
compartment, e.g. 111, from the
feed supply pressure of the higher pressure to the intermediate pressure of
that compartment, so as to
recover energy for driving a vacuum pump 103, which reduces the lower pressure
below ambient
pressure so as to enhance the PSA process performance.
FIGS. 5A and SB show the second stator valve face, at section 16'-1T of FIG.
1. Open ports
of the valve face are second valve function ports communicating directly to a
light product delivery
compartment 121; a number of light reflux exit compartments 122, 123, 124 and
125; and the same
number of light reflux return compartments 126, 127, 128 and 129 within the
second stator. The
second valve function ports are in the annular ring defined by circumferential
seals 131 and 132.
Each pair of light reflux exit and return compartments provides a stage of
light reflux pressure
letdown, respectively for the PSA process functions of supply to backfill,
full or partial pressure
equalization, and cocurrent blowdown to purge.
Illustrating the option of light reflux pressure letdown with energy recovery,
a split stream
light reflux expander 140 is shown in FIGS. 1 and SA to provide pressure let-
down of four light reflux
stages with energy recovery. Light reflux expander 140 provides pressure let-
down for each of four
light reflux stages, respectively between light reflux exit and return
compartments 122 and 129, 123
and 128, 124 and 127, and 125 and 126 as illustrated. The light reflux
expander 140 may power a
light product booster compressor 145 by drive shaft 146, which delivers the
oxygen enriched light
product to oxygen delivery conduit 147 compressed to a delivery pressure above
the higher pressure
of the PSA cycle.
Since the light reflux and light product have approximately the same purity,
expander 140
and light product compressor 145 may be hermetically enclosed in a single
housing, which may be
conveniently integrated with the second stator as shown in FIG. 1. This
configuration of a
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"turbocompressor" light product booster without a separate drive motor is
advantageous, as a useful
pressure boost of the light product can be achieved without an external motor
and corresponding shaft
seals, and can also be very compact when designed to operate at very high
shaft speeds.
FIG. 5B shows the simpler alternative of using a throttle orifice 150 as the
pressure letdown
means for each of the light reflux stages.
Turning back to FIG. 1, compressed feed gas is supplied to compartment 113 as
indicated by
arrow 125, while heavy product is exhausted from compartment 117 as indicated
by arrow 126. The
rotor is supported by bearing 160 with shaft seal 161 on rotor drive shaft 162
in the first stator 8,
which is integrally assembled with the first and second valve stators. The
adsorber rotor is driven by
motor 163 as rotor drive means.
As leakage across outer circumferential seal 131 on the second valve face 11
may
compromise light product purity, and more importantly may allow ingress of
humidity into the second
ends of the adsorbers which could deactivate the nitrogen-selective or CO-
selective adsorbent, a
. buffer seal 170 may be included to provide more positive sealing of buffer
chamber 171 between
seals 131 and 171. Even though the working pressure in some zones of the
second valve face may be
subatmospheric (in the case that a vacuum pump is used as exhauster 103),
buffer chamber is filled
with dry light product gas at a buffer pressure positively above ambient
pressure. Hence, minor
leakage of light product outward may take place, but humid feed gas may not
leak into the buffer
chamber. In order to further minimize leakage and to reduce seal frictional
torque, buffer seal 171
seals on a sealing face 172 at a much smaller diameter than the diameter of
circumferential seal 131.
Buffer seal 170 seals between a rotor extension 175 of adsorber rotor 4 and
the sealing face 172 on
the second valve stator 9, with rotor extension 175 enveloping the rear
portion of second valve stator
9 to form buffer chamber 171. A stator housing member 180 is provided as
structural connection
between first valve stator 8 and second valve stator 9. Direct porting of
adsorbers to the stator face is
an alternative to providing such seals and is described in commonly-owned, co-
pending U.S.
Provisional Application No. 60/301,723, filed June 28, 2001, and incorporated
herein by reference.
In the following system figures of this disclosure, simplified diagrams will
represent a PSA
apparatus or module. These highly simplified diagrams will indicate just a
single feed conduit 181 to,
and a single heavy product conduit 182 from, the first valve face 10; and the
light product delivery
conduit 147 and a single representative light reflux stage 184 with pressure
let-down means
communicating to the second valve face 11. Reference numerals pertaining to
PSA units as described
above will be unprimed for an oxygen enrichment PSA or VPSA unit, and primed
for a hydrogen
purification PSA or VPSA unit. Any type of gas separation device could be
substituted for the PSA,
including other types of adsorption modules or gas membrane separation
systems, although rotary
PSA systems currently are deemed preferred systems. The disclosed systems and
processes also
could be used with fuel cell types other than PEM fuel cells.
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FIG. 6 shows a fuel cell power plant 200 comprising a fuel cell 202, a steam
reforming fuel
processor 204, a hydrogen purification PSA system 205, and an oxygen
enrichment VPSA system
206. Fuel cell 202 comprises an anode channel 208 including an anode gas inlet
210 and an anode
gas outlet 212, a cathode channel 214 including a cathode gas inlet 216 and a
cathode gas outlet 218,
and a PEM electrolyte membrane 220. Membrane 220 cooperates with the anode
channel 208 and the
cathode channel 214 to facilitate ion exchange between the anode channel 208
and the cathode
channel 214.
The oxygen VPSA system 206 extracts oxygen gas from feed air, and typically
comprises a
PSA rotary module 1 and a compressor 101 for delivering pressurized feed air
to the feed
compartments of the rotary module 1. The oxygen VPSA system 206 includes a
vacuum pump 103
coupled to the compressor 101 for withdrawing nitrogen enriched gas as heavy
product gas from the
blowdown and exhaust compartments of the rotary module 1, and discharging the
nitrogen enriched
gas from conduit 225. The adsorbers 3 of rotary module 1 have a first zone 26
loaded with a suitable
desiccant such as alumina gel for substantially removing water vapor, and a
second zone 27 loaded
with a zeolite, generally nitrogen-selective zeolite. Dry oxygen enriched air
as the light product gas
of VPSA module 1 is delivered by conduit 147 to water management chamber 230
for humidification,
and thence by conduit 231 to cathode inlet 216. A portion of the oxygen reacts
with hydrogen ions
when electric current is generated, to form water in the cathode. The cathode
exhaust gas now
containing a reduced amount of oxygen (but still typically oxygen-enriched
well above ambient air
composition) is withdrawn from cathode exit 218 by conduit 232. A portion of
the cathode exhaust
gas is removed from conduit 232 by conduit 233 and flow control valve 234, and
may either be
vented to atmosphere for purging nitrogen and argon accumulations, or else
returned to the first valve
face 10 of PSA module 1. as a feed pressurization stream at an intermediate
pressure below the higher
pressure of the PSA cycle. The remaining cathode exhaust gas is supplied to
suction port 240 of an
ejector 242, which serves as cathode gas recirculation means. Ejector 242
receives enriched oxygen
from conduit 147 through nozzle 244, which drives recirculation of cathode
exhaust gas from suction
port 240, mixes the enriched oxygen and recirculating cathode exhaust gas
before pressure recovery
in diffuser 246 and delivers the combined oxygen enriched gas stream to water
management chamber
230 where excess water is condensed. The excess water is either exhausted
through valve 250, or else
is delivered as water reactant to fuel processor 204 by water pump 252 through
conduit 254.
A hydrocarbon fuel, supplied to the fuel processor 204 by a feed pump or
compressor 260, is
combined with water from conduit 254, and is vaporized and preheated in heat
exchanger 262. The
preheated stream of fuel and steam is then admitted to steam reforming
catalytic chamber 264, which
is heated by burner 266 whose flue gas heats the heat exchanger 262. In the
example that the fuel is
methane, the following steam reforming reactions take place:
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CHa + Hz0 -> CO + 3Hz
CHa+ 2Hz0 -~ COz + 4Hz
The resulting reformate or "syngas" (dry composition approximately 70% Hz with
roughly
equal amounts of CO and COz as major impurities, and unreacted CHa and Nz as
minor impurities) is
cooled to about 250°C, and then passed to the water gas shift reaction
zone 268 for reacting most of
the CO with steam to produce more Hz and COz:
CO + Hz0 -~ COz + Hz
The hydrogen rich reformate still contains about 1% to 2% CO after water gas
shift, along
with substantial amounts of carbon dioxide and water vapor. For high
performance and longevity of a
PEM fuel cell, it CO concentration should be reduced well below 100 ppm and
preferably below 10
ppm. Consequently, the impure reformate is admitted by conduit 270 to the
higher pressure feed port
of hydrogen PSA unit 205, including rotary PSA module 1'. Adsorbers 3' of
rotary module 1' have a
first zone 26' loaded with a suitable desiccant, such as alumina gel, for
substantial removal of water
vapor, a second zone 2T loaded with an adsorbent selective for CO removal, and
at least partial bulk
removal of COs, and a third zone 28' loaded with an adsorbent suitable for
further removal of CO and
at least partial removal of other impurities, such as Nz. There can be
numerous combinations and
variations of suitable adsorbents for the three zones of the hydrogen PSA
adsorbers, as already recited
above. These zones may be discrete, may have diffused boundaries, or in some
embodiments the
materials selected for each zone may be homogeniously moved.
Purified hydrogen light product from the hydrogen PSA module 1' is delivered
by conduit
147' to an ejector 242' which is a recirculation means for partial
recirculation of hydrogen rich anode
gas through fuel cell anode channel 208. The hydrogen rich gas from ejector
242' is delivered to
anode inlet 210, passed through anode channel 208, and then exhausted from
anode exit 212 in part
back to the suction inlet of ejector 242'. Recirculation of anode gas through
the ejector 242' is
optional, so this ejector may be omitted. The remaining portion of the anode
exhaust gas (or all of it
in the case that ejector 242' is omitted) is conveyed by conduit 280 back to a
feed pressurization port
in the first valve surface 10' of hydrogen PSA module 1', so as to retain
hydrogen within the system
while using the hydrogen PSA unit to reject impurities from the anode gas
loop. A larger fraction of
anode gas is recycled in this manner back to the PSA unit when adsorbent and
PSA process
combinations are selected that remove CO almost completely while allowing some
passage of other
impurities such as Nz and perhaps some COz. Conversely, only a small amount of
anode exhaust gas
is recycled back to the PSA to prevent inadvertent impurity accumulations,
when the adsorbents and
PSA cycle are designed to achieve high purity hydrogen with nearly compete
removal of CO and
other impurities as well.
Exhaust second product gas from the hydrogen PSA module 1' is exhausted from
valve face
10' by conduit 285 to burner 266.
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It will be understood by those of ordinary skill in the art that the hydrogen
PSA unit of this
invention, with the above specified combinations and variations of adsorbents
in the sequential zones
of the adsorbers, may be applied in conjunction with alternative fuel
processors, including partial
oxidation or autothennal reactors for processing of heavy as well as light
hydrocarbon fuels to
generate hydrogen rich reformate, from which CO and other impurities must be
removed.
FIGS. 7-9 show a fuel cell power plant 200 that includes a fuel cell 202, a
steam reforming
fuel processor 204, a hydrogen purification PSA system 205, and an oxygen
enrichment PSA or
10 VPSA system 206. Fuel cell 202 comprises an anode channel 208 including an
anode gas inlet 210
and an anode gas outlet 212, a cathode channel 214 including a cathode gas
inlet 216 and a cathode
gas outlet 218, and a PEM electrolyte membrane 220, Membrane 220 cooperates
with the anode
channel 208 and the cathode channel 2I4 to facilitate ion exchange between the
anode channel 208
and the cathode channel 214.
15 The oxygen PSA or VPSA system 206 extracts oxygen gas from feed air, and
comprises a
PSA module 1, typically a rotary module 1, and a compressor 101 for delivering
pressurized feed air
to the feed compartments of the rotary module 1. Nitrogen enriched gas as
heavy product gas from
the blowdown and exhaust compartments of the rotary module 1 is withdrawn by
conduit 182, either
for discharge directly by atmosphere as in FIG. 7 or to a vacuum pump 103 for
discharge as in FIG. 8.
20 The adsorbers 3 of rotary module 1 have a first zone 26 loaded with a
suitable desiccant, such as
alumina gel, for substantial removal of water vapor, and a second zone 27
loaded with a nitrogen-
selective zeolite. Dry oxygen enriched air as the light product gas of VPSA
module 1 is delivered by
conduit 147 to humidification chamber 230 and thence by conduit 231 to cathode
inlet 216. A
portion of the oxygen reacts with hydrogen ions when electric current is
generated, to form water in
25 the cathode. The cathode exhaust gas now containing a reduced amount of
oxygen (but still typically
oxygen-enriched well above ambient air composition) plus water is withdrawn
from cathode exit 218
by conduit 232 to separator 233.
In FIGS. 7 and 8, a portion of the humid cathode exhaust gas (or water
condensate) is
removed from separator 233 by conduit 234, which transfers water and any
recycle oxygen back to
humidification chamber 230 for recirculation through cathode channel 214. Any
oxygen recirculation
through conduit 234 must be driven by appropriate recirculation pressure boost
means, such as a
blower or an ejector.
If fuel processor 204 in FIGS. 7 and 8 is a partial oxidation or autothennal
reformer, the
remaining oxygen (plus any accumulated argon and nitrogen) and the fuel cell
product water are
delivered from separator 233 by conduit 235 to the fuel processor 204. This
delivery of cathode
exhaust to the reformer provides enriched oxygen to assist the partial
oxidation or autothermal
reforming process, together with water product of the fuel cell as vapor and
condensate, and also
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carries some fuel cell waste heat to assist in cooling the fuel cell stack
while preheating reactants to
the reformer. If fuel processor 204 in FIGS. 7 and 8 is a steam reforming
reactor, the fuel cell product
water condensate is delivered from separator 233 by conduit 235 to the fuel
processor 204. In that
event, accumulations of argon and nitrogen in the cathode channel 214 can be
recycled from separator
233 back to the oxygen PSA unit 1 as shown in FIG. 9 by conduit 236 to the
first valve face 10 of
PSA module 1 as a feed pressurization stream at an intermediate pressure below
the higher pressure of
the PSA cycle, or else purged to atmosphere.
A hydrocarbon fuel is supplied to the fuel processor 204 by a feed pump or
compressor 260,
combined with water from conduit 235, and vaporized and preheated in heat
exchanger 262. The
preheated stream of fuel and steam is then admitted to reforming catalytic
chamber 264. In the
example that the fuel is methane, the following steam reforming reactions take
place,
CH4 + Hz0 --> CO + 3Hz
CH4 + 2Ha0 --> COz + 4Hz
in addition to partial combustion in the case of an autothermal reformer:
CH4 + 1/20z -~ CO + 2Hz
The resulting reformate or "syngas" (dry composition approximately 70% Hz with
roughly
equal amounts of CO and COz as major impurities, and unreacted CH4 and Nz as
minor impurities) is
cooled to about 250°C, and then passed to the water gas shift reaction
zone 268 for reacting most of
the CO with steam to produce more Hz and COz:
CO + Ha0 -~ COz + Hz
The hydrogen rich reformate still contains about 1% to 2% CO after water gas
shift, along
with substantial amounts of carbon dioxide and water vapor. For high
performance and longevity of a
PEM fuel cell, it CO concentration should be reduced well below 100 ppm and
preferably below 10
ppm. Consequently, the impure reformate is admitted by conduit 270 to the
higher pressure feed port
of hydrogen PSA unit 205, including rotary PSA module 1'. As described above,
the adsorbers 3' of
rotary module 1' have a first zone 26' loaded with a suitable desiccant, such
as alumina gel, for
substantial removal of water vapor, a second zone 2T loaded with an adsorbent
selective for CO
removal, and at least partial bulk removal of COz, and a third zone 28' loaded
with an adsorbent
suitable for further removal of residual CO and at least partial removal of
other impurities, such as Nz.
Purified hydrogen light product from the hydrogen~PSA module 1' is delivered
by conduit
147' to anode inlet 210, passed through anode channel 208, and then exhausted
from anode exit 217
back to a feed pressurisation compartment in the first valve surface 10' of
hydrogen PSA module 1'.
This system retains hydrogen within the fuel cell anode loop, including
conduits 14T and 280, and
anode channel 208, while using the hydrogen PSA unit i'to reject impurities
that otherwise would
accumulate on the anode 208.
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Exhaust second product gas from the hydrogen PSA module 1' contains water
vapor, COa,
and combustible values including Hz, CO and any unreacted fuel from the
reformer. This gas is
exhausted from valve face 10' by conduit 285 to low pressure burner 290, where
this fuel is oxidized
completely, possibly over a suitable catalyst to ensure stable combustion of
this low BTU gas and to
suppress NOx formation. Burner 290 delivers hot products of combustion to heat
exchange channel
292, which is in countercurrent thermal contact for heat recovery to reformer
reactor zone 264 and
preheater zone 262. After cooling in channel 292 and further cooling in heat
exchanger 296, the flue
gas from burner 290 is discharged to atmosphere by exhaust conduit 294.
The hydrogen PSA module purifies the hydrogen so as to remove essentially all
contaminants deleterious to the fuel cell anode, including unreacted fuel
components such as
methanol, as well as incompletely reacted fuel components such as CO, and also
other contaminants
such as hydrogen sulphide and halogens that might originate with fuels such as
landfill gas. The
heating value of all such fuel byproduct impurities in the hydrogen is
recovered by combustion of the
PSA tail gas, to heat the fuel processor and/or an auxiliary thermal engine
cycle powering auxiliary
compression loads and possibly other mechanical loads. As methanol is harmful
to PEM fuel cells,
prior art methanol reformers for PEM fuel cells have been designed to achieve
very high conversion
to minimize methanol slip into the hydrogen-rich reformate gas, hence
requiring a large catalyst
inventory in a correspondingly large reactor vessel. The presently disclosed
processes and systems
allow methanol reformers to operate with relatively greater slip of methanol
info the refonnate gas
(syngas), as that gas will be purified by the hydrogen PSA module to remove
the methanol impurity
from the hydrogen and deliver it to the hydrogen PSA tail gas for recovery of
its heating value by
useful combustion. Hence, a methanol reformer can advantageously be designed
to operate at less
high necessary conversion of methanol, thus reducing the required methanol
reforming catalyst
inventory and reactor vessel size.
Fuel processor 204 is also thermally integrated with a high pressure burner
300, to which a
portion of the fuel from fuel pump 260 may be introduced by conduit 301.
Compressed air is
supplied to burner 300 from feed compressor 101 through conduit 302, heat
exchanger 296 (for
recuperative heat exchange from exhaust flue gas) and heat exchange channel
304, which is in
countercurrent thermal contact for heat recovery from water gas shift reaction
zone 268 and reformer
reactor zone 264 if the reforming reaction includes partial oxidation for net
exothermicity. Hot
products of combustion [including nitrogen and unreacted oxygen] from
combustion chamber 300 are
conveyed by conduit 310 to expander turbine 315, coupled by shaft 316 to
compressor 101. The
combination of compressor 101 and expander 315 are shown as a free rotor
turbocompressor 320,
similar to an automotive turbocharger. Alternatively a drive motor or a
generator may be coupled to
shaft 316, for starting, power assist, or net energy delivery. In FIGS. 7 and
8, a blower 330 driven by
motor 332 is provided to boost the inlet pressure to compressor 101, if
desired to assist the
compression of feed air in normal operation, but typically only as a starting
device to initiate rotation
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of turbocompressor 320, in which case bypass valve 334 is opened during normal
operation after
starting.
The still hot gas discharged by expander 315 is discharged by conduit 336 to
low pressure
burner 290, providing heat and oxygen to support catalytic combustion therein.
Supplemental air or
oxygen may be provided to low pressure burner 290 if required during starting
or any phase of normal
operation.
While FIG. 7 shows an embodiment whose lower working pressure is atmospheric,
FIG. 8
shows an embodiment with vacuum applied to the oxygen and hydrogen PSA units
to improve their
performance, perhaps to enable a reduced working pressure of the fuel cell. Of
course, separate
10- vacuum pumps could be provided for each of the oxygen and hydrogen PSA
units. Vacuum pump
338 receives the second product exhaust gases at subatmospheric pressure from
both the oxygen PSA
1 and the hydrogen PSA 1' by respectively conduits 182 and 182', and delivers
the combined stream
to the catalytic low pressure burner 290 by conduit 285. Vacuum pump 338 is
provided as a
turbocompressor 340 with expander 345 driving pump 338 through shaft 346.
Expander 345 is
arranged in parallel or series with expander 315 to expand hot gas delivered
by conduit 310 from high
pressure burner 300.
The combustion turbine embodiments for powering auxiliary compression
machinery have
the important advantage of using readily available and low cost turbocharger
equipment. FIG. 9
shows an alternative embodiment using a rotary internal combustion engine 400
to power the
compressor 101 and optional vacuum pump 103 of the oxygen PSA 206 by shaft
coupling 405, while
itself providing vacuum suction if desired for the hydrogen PSA 205. The
engine 400 may also power
any other compressors or vacuum pumps that may be provided for the hydrogen
PSA 205 as well as
any auxiliary devices such as an electric generator. Engine 400 is fuelled, at
least in part, by
hydrogen PSA tail gas, and has a starter motor 410 (or supplemental power
output generator 410).
Engine 400 may be any type of combustion engine such as an internal combustion
engine or
a combustion-enhanced turbocharger, but is here shown as a Wanleel engine.
Working chambers 412
are defined between rotor 414 and casing 415. The rotor is coupled to drive
shaft 405 by internal
gear 416. An intake port 421, exhaust port 422 and spark plugs 423 are
provided in casing 415. A
water cooling jacket 425 is provided. The engine has an air filter 426
delivering air to carburetor 427,
and to intake port 421. The carburetor mixes the air with hydrogen PSA exhaust
gas delivered by
exhaust conduit 182' to carburetor 427.
FIG. 9 shows details of an illustrative water management system. Product water
of fuel cell
202 is captured in separator 233 that includes a cooling coil 430, and is
delivered to liquid water
manifold 432. A portion of the water may be delivered from manifold 432 to
pump 435, and thence
by flow control 436 to the oxygen humidification chamber 230 and by flow
control 437 to engine
cooling j acket 425. Hot water from the engine cooling jacket is flash
evaporated and delivered
through depressurization orifice 485 and conduit 486 to methanol reforming
reactor catalyst zone
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264, which in turn is in heat exchange contact with the engine exhaust in
channel 440. Engine
exhaust is delivered from exhaust port 422 via conduit 442 to channel 440 for
exhaust heat recovery
to the endothermic methanol vapor phase reforming reaction in reactor zone
264, and then through
emission control after-treatment catalyst 443 and exhaust pipe 444 to
atmosphere.
Reformate hydrogen is delivered from reactor zone 264 by conduit 450 to feed
hydrogen
PSA unit 205. A portion of the reformate may be diverted to carburetor 427
from conduit 450 by
flow control 452 as supplemental fuel for engine 400.
A portion of the water condensate from separator 233 may be delivered via
conduit 434 by
pump 460 to liquid fuel mixing chamber 465, which also receives liquid
methanol fuel delivered by
fuel pump 260. The flow rates of pumps 260 and 460 are adjusted to achieve a
desired concentration
ratio of the waterlmethanol mixture exiting the mixing chamber 465 by conduit
466 delivering this
mixture as fuel cell stack coolant circulated.through cooling passage 468
through the fuel cell stack
202. The coolant pressure is maintained high enough to maintain it in the
liquid phase within the
cooling passage. The methanol present in the mixture may provide useful
antifreeze properties to the
coolant mixture. A portion of the water/methanol mixture coolant exiting
cooling passage 468 is flash
evaporated in separator 474 by depressurization valve 475 to approximately the
working pressure of
reforming reactor zone 264, and the resulting vapor mixture is delivered by
conduit 480 to the
reforming reactor catalytic zone 264. The balance of the water/methanol
mixture coolant is
repressurized and recirculated by pump 470 through cooling radiator 471 to
reject fuel cell stack heat
that has not been recovered to vaporize the water and methanol reactants.
Alternatively, a
water/methanol mixture could be delivered to the engine cooling jacket 425,
vaporized, and then
delivered to a reforming reactor.
If desired, the recovered water from the fuel cell could be delivered to only
the cooling
passage 468 or engine cooling jacket 425. Alternatively, water from an outside
source could be
delivered to cooling passage 468 and mixed with hydrocarbon fuel or water from
an outside source
could be delivered to the engine cooling jacket 425.
Alternatively, the embodiment of FIG. 9 may be adapted so that steam reforming
of
methanol vapor is conducted in the hydrogen PSA unit 205. The methanol
reforming reaction zone as
described above may be removed from channel 264 to the second zone 27' of the
hydrogen PSA unit
205. Channel 264 being heated by engine exhaust in channel 440 is used only to
preheat the reactant
mixture of methanol vapor and steam. In the hydrogen PSA unit 205, first zone
26' contains an
adsorbent selective for carbon dioxide in the presence of steam and methanol
vapor, e.g. promoted
hydrotalcite at a working temperature of the first zone at about 300°
to 450°C. Second zone 27'
contains the methanol reforming catalyst, e.g. Cu-ZnO, which is also active
for water gas shift, at a
working temperature of about 150° to 300°C. The third zone 28'
contains an adsorbent selective for
steam and methanol vapor, e.g. alumina, 13X or a hydrophobic zeolite such as Y
zeolite or silicalite,
at a working temperature of about 150° to 80°C . A hydrophobic
adsorbent can be more selective for
CA 02424615 2003-04-02
WO 02/35623 PCT/CA01/01523
methanol vapor than water vaopour, thus ensuring that a sufficiently high
steam/methanol ratio is
maintained throughout the reaction zone, and also perhaps allowing some slip
of water to humidify
the hydrogen product.
Accordingly, essentially 100% conversion and selectivity (equivalent to
substantially
complete removal of CO) are achieved in the reaction of steam methanol
reforming:
CH30H + Hz0 -> COz + 3liz
The systems shown in FIGS. 6-9 are only examples and other systems with
difference
arrangements of devices and conduits, or with additional or fewer devices and
conduits could also be
10 used.
